1. Field of the Invention
The present invention relates to a method of forming and immobilizing small particles on a filter substrate.
2. Description of Related Art
Small sized particles such as nanoparticles and particles below several micrometers are of great technological importance for water purification. Due to their small size and high surface area they have distinguishing properties, one of which includes being highly efficient. However, they are difficult to handle and recover in practice. It is difficult to immobilize these small particles on a substrate for a water filter.
Metallic silver nanoparticles may be synthesized within polyelectrolyte multilayers (PEM) on polystyrene tissue-culture substrates and quartz wafers in optical applications. PEMs are initially formed on the substrates and then the silver ions are diffused into the PEMs and reduced to silver metal nanoparticles. These nanoparticles are imbedded in the PEMs where they are immobilized.
Polyelectrolytes are polymers whose repeating units bear an electrolyte group. A repeating unit is the simplest structural entity of a polymer chain and defines polymer structure. A polymer consists of several repeat units linked together successively along the chain, in a similar manner to the beads of a necklace. A repeat unit, however, is not to be confused with a monomer, which refers to the small molecule from which a polymer is synthesized.
One of the simplest repeat units is that of polyethylene:—[CH2—CH2—]n—Polypropylene has the repeat unit:—[CH2—CH(CH3)]n—
The subscript “n” denotes the degree of polymerization or the number of units linked together. The molecular mass of the repeat unit, MR, is simply the sum of the atomic masses of the atoms within the repeat unit. The molecular mass of the chain is just the product nMR.
These groups will dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers (high molecular weight compounds), and are sometimes called polysalts. Like salts, their solutions are electrically conductive. Like polymers, their solutions are often viscous. Charged molecular chains, commonly present in soft matter systems, play a fundamental role in determining structure, stability, and the interactions of various molecular assemblies. Theoretical approaches to describing their statistical properties differ profoundly from those of their electrically neutral counterparts, while their unique properties are being exploited in a wide range of technological and industrial fields. One of their major roles, however, seems to be the one played in biology and biochemistry. Many biological molecules are polyelectrolytes. For instance, polypeptides (thus all proteins) and DNA are polyelectrolytes. Both natural and synthetic polyelectrolytes are used in a variety of industries.
Polyelectrolytes have several utility applications mostly related to modifying flow and stability properties of aqueous solutions and gels. For instance, they can be used to either stabilize colloidal suspensions, or to initiate flocculation (precipitation). They can also be used to impart a surface charge to neutral particles, enabling them to be dispersed in aqueous solution. They are thus often used as thickeners, emulsifiers, conditioners, flocculants, and even drag reducers. They are used in water treatment and for oil recovery. Many soaps, shampoos, and cosmetics incorporate polyelectrolytes. Furthermore they are added to many foods and to concrete mixtures (super plasticizer). Some of the polyelectrolytes that appear on food labels are pectin, carrageenan, alginates, polyvinylpyrrolidone and carboxymethyl cellulose. All but the last two are of natural origin.
Polyelectrolytes which are water-soluble have biochemical and medical applications such as using biocompatible polyelectrolytes for implant coatings and for controlling drug release.
Acids are classified as either weak or strong (and bases similarly may be either weak or strong). Similarly, polyelectrolytes can be divided into ‘weak’ and ‘strong’ types. A ‘strong’ polyelectrolyte is one which dissociates completely in solution for most reasonable pH values. A ‘weak’ polyelectrolyte, by contrast, has a dissociation constant (pKa or pKb) in the range of approximately 2 to approximately 10, meaning that it will be partially dissociated at intermediate pH. Thus, weak polyelectrolytes are not fully charged in solution, and moreover their fractional charge can be modified by changing the solution pH, counter ion concentration, or ionic strength.
The physical properties of polyelectrolyte solutions are usually strongly affected by this degree of charging. Since the polyelectrolyte dissociation releases counter-ions, this necessarily affects the solution's ionic strength, and in turn affects other properties, such as electrical conductivity.
When solutions of two oppositely charged polymers (that is, a solution of polycation and one of polyanion) are mixed, a bulk complex (precipitate) is usually formed. This occurs because the oppositely-charged polymers attract one another and irreversibly bind together.
Polyelectrolyte multilayers are thin films constructed using a layer-by-layer (LbL) deposition technique. During LbL deposition, a suitable growth substrate (usually charged) is dipped back and forth between dilute baths of positively and negatively charged polyelectrolyte solutions. During each dip a small amount of polyelectrolyte is adsorbed and the surface charge is reversed, allowing the gradual and controlled build-up of electrostatically cross-linked films of polycation-polyanion layers. Scientists have demonstrated thickness control of such films down to the single-nanometer scale. LbL films can also be constructed by substituting charged species such as nanoparticles or clay platelets in place of or in addition to one of the polyelectrolytes. LbL deposition has also been accomplished using hydrogen bonding instead of electrostatics.
Multilayer formation via layer-by-layer deposition of alternating charged polyelectrolytes requires a strong short-range attraction between the two types of polymer chains for the formation of multilayers. For strong enough short-range attraction, the adsorbed amount per layer increases (after an initial decrease), and finally stabilizes in the form of a polyelectrolyte multilayer that can be repeated hundreds of times. For weak short-range attraction between any two adjacent layers, the adsorbed amount (per added layer) decays as the distance from the surface increases, until the chains stop adsorbing altogether. The dependence of the threshold value of the short-range attraction as function of the polymer charge fraction and salt concentration is calculated.
The main benefits to PEM coatings are the ability to conformably coat objects (that is, the technique is not limited to coating flat objects), the environmental benefits of using water-based processes, reasonable costs, and the utilization of the particular chemical properties of the film for further modification, such as the synthesis of metal or semiconductor nanoparticles, or porosity phase transitions to create anti-reflective coatings, optical shutters, and superhydrophobic coatings.
There is no standard method for immobilizing particles below several micrometers on substrates for antimicrobial applications for water purification. Some current methods may be implemented such as sintering, plasma, and electrospraying where small particles are deposited on the substrate. However, these methods usually require extreme conditions such as a high temperature or high voltage.